1. Field of the Invention
This invention relates to improvements in heart pacer circuits, and in oscillators for use therein, and, more particularly, to improvements in heart pacer circuits of the type which supply stimulation pulses in the absence of naturally occurring heart pulses. 2. Description of the Prior Art
In the production of heart pacers, devices which produce electrical pulses for application to a heart to stimulate its beat, much emphasis has been recently placed upon achieving a heart pacer of relatively small size to facilitate its implantation into the body of the user. This has resulted in increased emphasis upon achieving circuitry of high reliability and quality, since once the heart pacer device is implanted, repairs although mechanically simple, may involve surgical operations just to achieve access to the device.
The heart pacers commonly used are of three general classifications, fixed rate, synchronous, and demand pacers. The fixed rate pacer produces stimulation pulses at a particular fixed rate, such as 72 beats per minute, at its own internally controlled rate, regardless of the occurrence of natural heart pulses. Synchronous heart pacers, on the other hand, produce stimulation pulses in synchronism with a particular natural heart pulse, such as a predetermined time after the production of a heart "P" wave. Demand heart pacers produce heart stimulation pulses only in the absence of a particular heart pulse. Typically, the demand heart pacers are activated by the absence of a naturally occurring "R" wave, since the "R" wave is the largest heart produced wave, and easiest to detect. Thus, after a demand heart pacer has been installed, if the wearer's heart fails to produce an "R" wave, which would have caused the heart muscle to naturally contract, the demand pacer produces an artificial pulse which is applied to the heart via a conductive lead physically attached to the heart to induce the desired heart beat or contraction.
Ordinarily, demamd heart pacers are operated in the so-called "inhibit" mode, the output pulse generated by the demand heart pacer being inhibited by the heart producing its own naturally occurring heart pulse, although demand heart pacers are usually operable in a "fixed rate" mode under certain conditions. For example, typically demand heart pacers have a built in switch activatable from outside the user's body, such as by exposure to a magnetic field, or the like, to cause the heart pacer to switch from its "inhibit" mode to the "fixed rate" mode. Also, other naturally occurring conditions typically cause the demand heart pacer to switch to a "fixed rate" mode, such as signals or noise received at an undesirable frequency, which interfere with the pacer's detection of the naturally occurring heart pulses.
Because heart pacers are typically implanted, they ordinarily have as an integral part a power supply of one or more batteries to power the electronics to produce the heart stimulation pulses. Ordinarily, one or more chemical batteries are used; however, because of the relatively short useful lifetimes of chemical batteries, nuclear batteries have been recently considered for power supply sources, to increase the useful lifetime of the heart pacer.
One of the requirements, therefore, evident in the circuitry for use in conjunction with either chemical or nuclear batteries, is that the circuitry have power dissipation as low as possible. Furthermore, because nuclear batteries operate at relatively low voltages, the particular circuitry used with such batteries must be operable at such low voltages, and draw relatively low current, as well.
This introduces a particularly crucial design consideration in the heart pacer circuitry, that of having a quiescent state between heart pulses detected or produced which draws the minimum possible amount of current. For example, it has been proposed that in such quiescent state, as many as possible of the operable devices, such as transistors, or the like, be biased into nonconduction so as not to dissipate any power, while not actually producing stimulation pulses.
Another difficulty resulting from the pacer being implanted is in determining the state or condition of the power supply. For example, after the pacer has been implanted for an extended time, there has been no way of directly determining the condition of the power source, other than by monitoring the pulse production rate. Typically, the pulse production rate of the "fixed rate" oscillator of the pacer is designed to exhibit a lower pulse production rate at lower power supply voltages. However, this method is susceptable to inaccurate indications since components within the pacer circuit particularly the timing resistors and capacitors of the multivibrator circuit, may exhibit changes in their electrical values with age, such component value drifts producing increased or decreased heart production frequencies, resulting in possible erroneous indications of the battery condition.
Additionally, one of the problems presented in the prior art heart pacers is that the heart stimulation pulse decreases in power as the battery voltage decreases. Typically, the duration of the heart stimulation pulse produced is maintained at a constant level, therefore, the electrical power within the heart stimulation pulse envelope decreases as the battery voltage decreases. This is undesireable since it may necessitate replacement of the pacer at a time when pulses are being produced timely, but of power insufficient to stimulate heart contractions.
In addition to the power requirements, heart pacers must be unaffected by noise both from outside sources, and from the heart itself. For example, the heart pacer must be capable of distinguishing high frequency noise generated, for instance, by a nearby automobile ignition, from the electrical pulses of interest naturally occurring or generated within the heart in order that the pacer is not inhibited erroneously under the influence of the undesireable noise. The heart itself generates undesireable noise, which, although inconsequential to the heart organ, is undesireable for detection by the pacer. This heart noise, therefore must be distinguished, particularly after each heart beat stimulated by the pacer or naturally occurring. To distinguish this noise, typically, heart pacers have elements to time a "refractory period" built into their circuitry to block the detection of undesireable heart pulses occurring within that period. One of the difficulties encountered, however, is that in most commercially available pacers, if a noise signal is impressed upon the pacer at a frequency approximately twice that of a natural heart beat, just beyond the termination of a refractory period (i.e., one noise pulse falling within the refractory period, a subsequent pulse falling just outside the refractory period), the heart pacer will detect the second heart pulse and interpret it as a naturally occurring heart pulse, causing the stimulation pulse to be inhibited, whether it should be or not.